Wood in its natural state, in a living tree or newly cut lumber for example, often has a high moisture content with the moisture content varying depending on the particular type and location of the wood and the type and condition of the tree. The moisture is made up of bound water, that is, water bound within the cell walls and bound to carbohydrates and lignin polymers that are components of the cell walls, and free water, that is, water in the lumens. The lumens also often contain sap and other solutes in the free water. Although there is great variation between tree species it is not unusual for moisture content to be in the vicinity of 160%-200% (of the oven-dry weight of the wood).
For utility purposes, unless it is dried (a process known as seasoning), wood with a high moisture content, herein ‘green wood’, has undesirable properties including instability during drying and susceptibility to deterioration. Changes in the dimensions of wood due to changes in moisture content create significant problems when used in construction. It is therefore common practice to dry green wood in an attempt to produce a stable, practical material and to maximise its utility value. Because of the non-uniformity of wood and its high moisture content, it is not uncommon for the drying of wood to result in distortion of the wood or to cause damage to the wood structure such as warping and internal and surface checking.
The two common techniques used to dry green wood are air-drying and oven- or kiln-drying.
In air-drying, green wood is left to dry passively in the ambient air. This technique is climate dependent and is generally a slow process. The advantage of passive air-drying is simplicity and that it is a mild process (relative to kiln-drying) with the wood material not being subjected to high temperatures and high forced internal moisture gradient stress that may occur in kiln-drying. In oven- or kiln-drying, green wood is placed in an insulated chamber within which heated air is circulated. While this technique overcomes the disadvantage of air-drying namely being slow, it can result in other undesirable effects, such as kiln stains, which are dark coloured regions that are formed on the wood, and higher internal moisture gradient stresses (for instance the outside of the wood dries while the inside remains wet) that can cause a higher rate of checking or warping of the wood. These effects detract from wood quality, yield and value.
Changes in the dimensions, strength and flexibility of wood occur when the wood's cell walls lose bound water. Moisture gradient stress often occurs in conventional drying when parts of the wood (often the wood close to the surfaces in air or kiln drying) lose moisture from the lumens and from the cell walls at a greater rate than water is lost from the cell walls and lumens of other parts of the wood. Imaging of wood during the drying process when being air or kiln-dried shows the wood with dry edges and a wet core. The results are changes to the dimension and strength of some parts of the wood at rates and extents different to the changes in other parts of the wood. The result is often damage to the structure of the wood and distortion.
Techniques have been used to dry wood other than passive air-drying and kiln-drying. These include dehumidification, the use of azeotropes to distil water from green wood at temperatures below that of the boiling point of water, and also freeze-drying, where the water in green wood is frozen and subsequently removed by sublimation process. However, these techniques also create moisture gradient stress (although sometimes with different patterns of moisture distribution) and cause damage to cell walls and wood.
It is also known to dry wood by electromagnetic radiation drying such as radio frequency drying and microwave drying. Radio frequency vacuum ('RFV') drying in particular, results in less moisture gradient stress than air or kiln drying but these methods are time consuming and requires high energy input particularly when the wood has high moisture content. They are, however, more energetically viable and quick when used on wood with reduced moisture content, for instance, wood at or close to fibre saturation point.
Supercritical fluids are fluids that exhibit properties of both gas and liquid when subject to temperatures and pressures above those of the critical point of the fluid. A fluid in a supercritical state thus has the solvating abilities of a liquid, but with a gas-like diffusivity.
The use of supercritical fluids in wood processing other than for wood drying purposes is known. In U.S. Pat. No. 6,638,574, supercritical fluids are used for impregnating preservatives into wood, while in U.S. Pat. No. 4,308,200, supercritical fluids are used for extracting organic substances from wood. The known methods as described in the prior art utilise the solvating qualities of supercritical carbon dioxide and have been applied to carry timber treatments (insecticides etc.) into conventionally pre-dried timber, MDF board, laminated veneers etc, or carry organic substances out of pre-dried particulated wood.
In U.S. Pat. No. 5,041,192, in particular in column 4 lines 14-18, it is briefly noted that supercritical fluids can be used for drying wood in specialty applications. A possible specialty application is the process for drying of archaeological samples developed by the University of St Andrews, Fife, Scotland that includes drying archaeological wood by substituting the water in archaeological wood with methanol and then extracting the methanol using supercritical carbon dioxide.
U.S. Pat. No. 4,995,943 describes the use of carbon dioxide under super-atmospheric pressures of several atmospheres to further pre-treat and dry particulated cellulosic material sourced from naturally (air) dried feedsocks (branches, stalks) to prepare or enhance the particulate for further chemical treatment/conversion.
Persons skilled in the art view the use and applications of supercritical carbon dioxide (as a solvent for instance) as similar or equivalent to hexane chemistry. It is therefore seen as a “dry” chemistry. As a result, the use of supercritical carbon dioxide in relation to wood has uniformly been in relation to pre-dried wood or wood wherein the water content has been substituted by an organic solvent such as methanol.
It is an object of the present invention to either provide an improved process for the removal or water and solutes from the lumens of green wood, or to provide a wood product wherein the lumens of the wood are evacuated of water and solutes while the cell walls remain in their green state and with reduced moisture gradient variation throughout the wood, or to provide an improved process for drying and/or treatment of wood or at least provide the public with a useful choice.